


It's All Dean's Fault

by orphan_account



Series: Brothers and Twins College AU [1]
Category: Supernatural
Genre: A touch of emotional infidelity, Alternate Universe - College/University, Always Female Castiel (Supernatural), F/M, Happy Ending, M/M, Multi, Polyamory, Sibling Incest
Language: English
Status: Completed
Published: 2017-07-31
Updated: 2017-07-31
Packaged: 2018-12-09 13:07:21
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,454
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11669727
Author URL: https://archiveofourown.org/users/orphan_account/pseuds/orphan_account
Summary: Sam could not imagine a worse way to meet his new college roommate than to have him walk in on Sam having sex with his brother. His roommate as a few secrets of his own though, not to mention a hot sister.





	It's All Dean's Fault

**Author's Note:**

> I never thought I would write this ship. It only happened because I read my friend's fic [HERE](https://archiveofourown.org/works/11481993).

Dean pulled Sam against him by the hem of his shirt. “Wanna christen your bed?”

Sam laughed, resting his arms around Dean’s neck. “My roommate could be here any moment.”

Dean slid his hands into the back pockets of Sam’s jeans and pulled their hips close. “I’ll make it quick.” He kissed Sam’s neck.

Sam laughed when Dean’s stubble brushed his neck but sighed when Dean pulled down his t-shirt to graze his teeth across Sam’s collarbone.

“Is that a yes?” Dean asked, his lips against the hollow of Sam’s throat.

Sam groaned, half in frustration, half in pleasure, and walked backwards till his knees hit the edge of the freshly made dorm room bed. He fell back, tugging Dean onto the bed with him between his spread legs.

“Yeah alright, fuck me. But make it quick.” Sam smiled and pulled Dean down for a kiss.

Dean’s pants were low on his thighs and Sam was naked from the waist down except for his socks when Sam’s roommate walked in.

“Shit!”

“Oh, shit!”

“Uhh…”

Dean sat up pulling on his pants. Sam scrambled for a blanket.

“We’ll just give you a sec.” The guy backed out of the room, an embarrassed smile on his face.

“I thought you locked the door!” Sam hissed as soon as it was shut.

Dean threw Sam his pants. “I thought it locked automatically!”

“He has the key, obviously,” Sam said pulling his pants on angrily. “I thought you threw the deadbolt!”

“He couldn’t’ve knocked?” Dean argued, spreading his hands.

“To go into his own room?” Sam sighed. “Great, this is just great. Perfect way to meet the guy I have to live with for the next nine months, let him walk in on me with a dick in my ass. Hope he’s not homophobic.” He glared at Dean and went to open the door.

Sam looked at Dean to check if everything was in order and noticed his boxers on the floor. He looked at Dean and then at the floor significantly.

Dean noticed and kicked them under the bed.

Sam frowned, flattening his eyebrows into a straight line and looked at Dean unimpressed.

Dean gave an adorable half smile and shrug.

Sam pasted on a smile and opened the door. “Hi, I am so sorry about that. I’m-”

Sam hadn’t gotten a good look at the guy before, having been buried under Dean at the time, but he got one now. The guy was tall, shorter than Sam but still taller than most. He had tousled black hair and bright blue eyes set in a pale face with an angular jaw. He was undeniable attractive. There was also an equally attractive, nearly as tall, young woman standing next to him with the same thick dark hair and blue eyes.

Sam lost his train of thought.

“Don’t worry about it,” the guy laughed pushing through the door. He held out his hand. “I’m Jimmy. This is my sister Cas.”

Sam took it. “Sam, that’s my bro-”

“Dean,” he said, stepping up and extending his hand. “The boyfriend. And uh, sorry about that. You can blame me.”

Sam shot Dean a panicked look, realizing what he almost said. He was so used to introducing Dean as his brother.

The woman stepped up to Sam and extended her hand, distracting him. “Hi,” she said with a shy smile, but her grip was firm.

“Cas, right?”

“Correct. Sam?”

Sam nodded and let go of her hand. There was something about her smile and the sparkle in her pale blue eyes that drew Sam in. He couldn’t help but smile back.

“You were “breaking in” the mattress?” She gave finger quotes in the air and smirked at him.

Sam blushed and rubbed the back of his neck, “Sorry about that.”

She laughed. “It’s fine. Funny actually. It’ll be a fun story to tell people after we graduate.”

“We?” Sam asked.

Jimmy came over and slung an arm over her shoulder. “Did I mention she’s my twin. Smarty pants could have gone to Harvard or something but she deigned to go to a state college with me.” He squeezed her in a one armed hug and she looked at him adoringly.

Dean ruffled Sam’s hair like he was twelve years old still. “Sammy, here is the same way. You kids will get along great.”

Sam scowled and straightened his hair. “So what’s your major?”

“History,” Cas replied. “I haven’t decided on a concentration yet though.”

Sam looked at Jimmy.

“Philosophy, but I’m thinking of switching to engineering,” Jimmy said.

“Today,” Cas added with equal parts exasperation and affection.

Jimmy gave a rueful grin. “I haven’t really decided anything yet. What about you guys?”

“English Lit,” Sam answered.

Jimmy and Cas looked at Dean, waiting for his answer. He laughed. “Oh no, I’m a mechanic. He got all the brains in the- uh-” Dean cleared his throat and started again. “He’s got the brains in the relationship.” He finished with a weak smile.

Sam cringed internally. This was going to be hard. They normally said they were brothers and left out the part where they had sex. It was going to be tricky remembered to play it the other way around.

“Well, uh, we’ll let you get unpacked.” Sam headed for the door with Dean on his heels. “I’ll see you later.”

“Yeah, see you later,” Jimmy said. “And Sam?”

Sam turned back.

“Put a sock on the door knob next time.” Jimmy laughed.

Sam’s face turned bright red again and he tried to stammer out another apology.

“It’s fine. I’m just giving you shit, dude. Get out of here.”

Dean grinned and dragged Sam out of the room by his arm.

“I like him,” Dean commented once the door had shut.

“That’s because he’s as big a jerk as you,” Sam said, rolling his eyes.

Dean gave a look of mock offense.

“But maybe it’ll mean I’ll get along with him.”

***

It turned out Sam and Jimmy did get along pretty well. Jimmy was a little more eccentric and off the wall but he had the same teasing sense of humor Sam loved in Dean.

A few weeks into the semester, Sam got out of his College Writing class early. He was anxious to get back to his room to study for a Calculus test he had coming. When he got back to his room however, there was a sock on the door knob. Jimmy had never had anyone over before and hadn’t mentioned meeting someone. He could just want some personal time Sam figured but a loud distinctly female moan and some rhythmic knocking started up. Sam was dying with curiosity. He squashed it for now though and went to the library to study. He could wait till that evening to ask Jimmy who he’d been with.

When he got back to the room, Cas was helping Jimmy with some history homework. Sam didn’t know if Cas knew about Jimmy’s new friend so he had to wait till she left.

Cas spent a lot of time in their room. Her parents had made her go to an all girls dorm that didn’t allow male visitors but they could only keep the boys out, not the girls in.

Sam sat on his bed with his laptop, watching some over the top comedy. He needed a break from homework. The movie wasn’t that interesting though and he caught himself staring at Cas out of the corner of his eye.

He was fascinated the way her long lashes nearly brushed her cheek when she blinked and the warm smile curving her lips as she patiently explained something. She was beautiful. Her long dark hair such a contrast against her pale skin and Sam suddenly wondered what it would look like fanned out on a pillow beneath him, a flush of pink darkening her cheeks.

Cas chose that moment to look over at him. She waved her hand to get his attention.

Sam blinked and pulled his headphones off. “Yeah?”

“You were staring off into space.” She cocked her head to the side, considering him. “Are you not feeling well? You look flushed.” She got up from Jimmy’s bed and went to sit beside Sam. She pressed a soft hand against his forehead. “You feel like you might have a low fever.”

Sam could smell the clean floral scent of her skin and as she leaned over him he got a view of the curve of her breasts down the top of her shirt.

He pulled away quickly. “I’m fine.”

Shutting his laptop, Sam slid off the bed away from Cas. “I’m going back to the library,” he mumbled gruffly, gathering his things.

As he darted out of the room, he caught a quick indecipherable look from Jimmy. 

Guilt burned in Sam’s gut as he walked away. Guilt for fantasizing about his roommate’s sister and guilt for being mentally unfaithful to Dean. He didn’t think Dean would actually care he was thinking about having sex with someone else, he would probably take it as a playful challenge to fuck them out of Sam’s mind. The worst of the guilt was coming from the feeling it was more than just thinking she was hot.

Sam made sure to stay away longer this time. Long enough that hopefully Cas had left and Jimmy was asleep.

Only one managed to be true. Jimmy was sitting at his desk and turned to face Sam as he walked in.

“What was that earlier?” Jimmy asked.

Sam feigned ignorance. “What was what?”

“Dude, you were staring at me and Cas for like five whole minutes then got up and ran out of here with barely a goodbye.”

“I don’t have to tell you all my plans,” Sam said defensively, setting his stuff down and starting to get ready for bed. Then remembered. “And you don’t tell me everything either.”

Jimmy frowned. “What’s that supposed to mean?”

Sam scoffed. “It means you never told me you had a girlfriend.”

Jimmy stared a moment, scrutinizing Sam. Finally, he looked away. “So you- That’s- It’s not- I’d tell you if I could,” he finally got out.

Sam paused, curiosity overriding his anger. “What does that mean? Is it a professor or something?”

Jimmy shook his head. “I won’t ask you why you freaked out earlier, if you don’t ask me about this. Deal?”

Sam sighed and got into bed. “Fine.”

Jimmy got up and stripped down to his boxers for bed, the muscles in his back moving gracefully under the skin as he pulled the shirt over his head. Sam could appreciate that his roommate certainly was attractive. Whatever girl he found was lucky.

***

The next Friday night Sam sat at a table in a secluded corner of the library. Dean had said he had to cover for someone so Sam was taking the opportunity to study. He had headphones on and was trying to memorize the names and functions of the parts of a cell.

He felt a tickle on his arm. He looked up and jumped.

Cas was sitting serenely in the chair next to him.

He hit pause on the music and pulled off the headphones. “Jesus, Cas. You came out of nowhere. You trying to give me a heartattack?”

She blinked at him. “If you had your music set lower, you would have heard me. I greeted you before I sat down.”

“Yeah, I guess.” Sam took a deep breath trying to get his heart rate to slow. “Are you here to study?”

“I’m escaping actually. I was supposed to be having a girls night with some friends.” Cas wrinkled her nose. “My expectation was a girls night would be about girls. I had hoped I could discuss with them feminist ideals and women’s role in society throughout the years. They wanted to dress in skimpy clothing and attempt to attract men.”

Sam tried to hide his smile. “You need to get some new friends, Cas.”

“Yes. Hael and Anna are quite a bit different than me. And Naomi gets mad when I don’t go along with her plans like everyone else. She calls me weird.” Cas considered. “You could be my friend.”

Sam laughed at her matter of fact tone but sobered at her serious expression. Her intense blue eyes bored into his, waiting for his answer. Sam felt a tension stretch between them.

“I’m already your friend, Cas,” Sam said softly.

She finally blinked and looked away. “You’re my brothers friend. Not mine. That’s how it always is.”

Sam grabbed her small pale hand in his, the size difference making her seem smaller and frailer than she was. “Jimmy’s my roommate. You’re my friend.”

Cas looked up hopefully. “So we can spend time together, just the two of us?”

Sam swallowed. Cas didn’t use a lot of slang but “spend time together” felt like it had more significance than just hanging out. Maybe he was just projecting his own feelings. He shouldn’t let that hurt Cas.

He smiled. “Yeah, sure. Wanna do something right now?”

Cas brightened. “Can we discuss feminism and how women are viewed in society? I made note cards.”

Sam huffed a laugh. He squeezed Cas’s hand before letting go. “I’m sure my gender studies professor will appreciate it.”

It was well after one in the morning when Sam walked Cas back to her dorm. She assured him it was unnecessary but he wanted to anyway. Sam walked backed to his room with a bounce in his step. Cas was odd but so smart. He really loved talking to her.

Sam expected Jimmy to be down the hall in Chuck and Gabe’s room playing video games or something when he got back. He was surprised when Jimmy was still in the room and even more surprised when he noticed Dean cozied up next to him. Jimmy had his laptop on his knees and Dean was leaning in to see some video. They were laughing together like old friends.

“Hey, Sammy! Finally finished studying?” Dean looked up, laughter still crinkling the corners of his eyes.

Sam wasn’t exactly sure why anger and jealousy suddenly burned in his gut but he was pretty sure it had to do with the stupid grin on Dean’s face and the complete lack of space between Jimmy and Dean’s hips.

“What are you doing here?” Sam snapped.

Dean’s face fell. “I thought I’d surprise you this weekend. But Jimmy said you were studying so I didn’t wanna interrupt.”

Sam turned his glare on Jimmy. “So glad he could entertain you,” he said, sarcasm dripping heavily. He flung his bag down. “Did he tell you who he’s fucking? Because he won’t tell me.”

Jimmy and Dean exchanged a look which only pissed Sam off more. Jimmy got up, heading for the door. “I’ll give you guys some space.”

“What crawled up your ass and died?” Dean asked once the door shut.

“Nothing. Never mind. Let’s just go to bed,” Sam grumbled.

Dean gestured towards the door. “What about Jimmy?”

Sam spun. “What about him, Dean? You worried about him? You two were all cozy when I walked in.”

Dean rocked back on his heels in surprise. “Are you jealous?”

“No.” Sam said quickly.

Dean slid up behind Sam and wrapped his arms around him, kissing his shoulder. “Aww come on, Sammy. You know there’s no one else for me.”

Sam groaned, leaning back. “I know.”

Dean gripped his hips tightly, mouth moving up Sam’s neck. “Then why are you so pissy?”

Sam sighed. Dean would never do that to him. And neither would Jimmy. What was his problem? “I don’t know.” Sam pulled away. “Can we just go to sleep?”

Dean shrugged, his shoulders sagging lower in disappointment. “If that’s what you want.”

Sam sent Jimmy a quick text to tell him everything was fine and they were going to sleep.

The dorm mattress barely fit Sam alone so it was a tight fit when Dean visited. They always managed to make it work though sleeping close together on their sides.

Dean wrapped one arm around Sam’s waist, the warmth reassuring and steady. He let himself relax. Dean was here in bed with him and that’s all that mattered. Dean’s breathing slowed and evened and Sam started to drift off. Blue eyes and soft curves were waiting to welcome him as he fell asleep.

***

Things went mostly back to normal after that weekend. Sam made it a point to spend time with Cas apart from Jimmy. They still hung out with all three of them, four if Dean was around, but Sam always looked forward to when it was just him and Cas. He also noted frequent dirty jokes and heated looked between Jimmy and Dean. Sometimes it burned him up with jealousy though the more he thought about it the more it just turned him on. He had no romantic interest in Jimmy but he couldn’t deny it would be hot to watch the two of them together.

Cas was still the only girl Sam ever found in their room but Sam kept to the agreement to not ask about it and Jimmy didn’t ask too much about Sam spending more time with Cas.

As finals drew closer, Sam was studying in the library between classes when Cas appeared again.

“I like you Sam,” she said without blinking.

“Uh, I like you too Cas. What’s up?” Sam asked puzzled.

“No, I-” she sighed and raise her hands for air quotes. “Like-like you.”

Sam’s stomach flip flopped and his heart skipped a beat. “Uh-”

“I know you have Dean and I don’t even know if you’re into girls anyway. I just thought you should know.” She shrugged. “Since we’re friends and all. We are still friends, right?” She caught him with her piercing blue eyes.

Sam nodded. “Yeah,” he got out.

“Then one more thing. If we’re friends, I want to be totally honest. Jimmy and I are together. I was the girl you heard in his room a while back. Jimmy thought it best to keep it from you because most people think it’s wrong because we’re twins. But you’re my friend and friends tell each other secrets right?”

“Um-” Sam swallowed.

“Oh! I’m gonna be late for class. See you later friend!” Cas brushed her hand over his shoulder as she left.

“Hey, w-” Sam released a breath he’d been holding. She was already gone. He shook his head smiling. She liked him? But, she was with Jimmy? Obviously Sam didn’t have a problem with that...except what did it mean for him and Cas? It didn’t matter if he didn’t talk to Dean first. It was long overdue anyway.

Sam rubbed the back of his neck. Things were getting complicated.

***

Sam managed to avoid Jimmy, coming back late and leaving early till Friday when he could talk to Dean in person. He asked Dean if they could talk in the car as soon as he pulled up.

“Sure thing, Sammy.” Dean grinned “We gonna make out for old times sake?”

He didn’t try anything though once they were both in the car, just waited for Sam to talk.

“Cas told me something the other day.”

Dean snorted. “Dude, she says all sorts of crazy shit. What was it this time?”

Sam gave a short laugh. “Yeah, she said…” Sam debated where to start. “She said she liked me and… it made me realize, well I’ve kinda thought it for a while, but when she said it I couldn’t lie to myself anymore. I kinda like her too, you know like, more than friends.”

Dean was quiet for a moment. “You guys are just realizing this now?” he finally asked. “Jimmy and me talked about it weeks ago. Said he’s never seen Cas as comfortable with anyone as she is with you. Said she might like you and would that bother me. He even hinted you might like her too.” Dean scoffed. “As if I hadn’t noticed. Know what I said to him?”

Sam shook his head.

“I said, Sam means more to me than anyone else on this whole planet ever will. And I know Sam feels the same about me. That’s never gonna change.”

Sam made a small noise, tears filling his eyes at how much he must have hurt Dean. He tried to apologize but Dean cut him off.

“But- if there was someone else, someone that you cared about and who cared about you. As long as they understand that… well I know where I stand.”

In the space of a heartbeat, Sam flung himself across the car, climbing into Dean’s lap despite the fact that there was no room and he was way too big for this now.

He kissed Dean, trying to show him with lips and tongue how much he loved him, how grateful he was for everything Dean had ever done for him.

“I love you,” Sam murmured when he paused to breathe. Just in case it wasn’t obvious.

“I know,” Dean grinned and pulled him back down.

Eventually Sam crawled back to his side, back sore from where it had been hitting the steering wheel.

“So did Jimmy mention anything else when you guys discussed me and Cas?”

Dean frowned. “No.”

“Well, I think it’s time we told them we’re brothers,” Sam said casually.

Dean choked. “Why would we do that?”

“Because, remember how I mentioned Jimmy was sleeping with someone but he wouldn’t tell me who?”

Dean let out a surprised laugh. “You’re kidding?”

Sam shook his head. “Nuh uh. It was Cas.”

Dean laughed for real. “What are the chance’s? Well I guess we don’t have to worry about them hating us for it. But what does that mean for you and Cas?”

Sam shrugged. “Guess we’ll find out.”

***

Jimmy tensed when Sam opened the door, Dean trailing behind him.

“Look, Sam I know it’s weird. You can get a new roommate next semester but please be nice to Cas. She doesn’t always get why people think it’s a bad thing.

Sam didn’t say anything. He walked over and handed Jimmy two small plastic cards.

Jimmy looked at them, frowned and looked up. “Why are you showing me your licenses?”

“Look closer,” Dean suggested.

Jimmy met Dean’s eyes before looking down again. He paused. “Wait, are you guys married? How have I never asked what your last name was Dean?” Jimmy looked between the two of them.

“We aren’t married, we’re brothers. We told you we were dating because you walked in on us.” Sam scowled at Dean a little over the memory. It had been his idea after all.

Jimmy still looked confused. “But you are dating, or together or whatever?”

“Yes. Dean is my brother, like biological, same parents and everything, brother. And also…” Sam walked back to take Dean’s hand. “So much more.” Sam leaned in for a kiss.

Dean was embarrassed by the display but allowed it. Squeezing Sam’s hand in return.

They turned back to Jimmy for his reaction.

Jimmy stared at them for a second. “Guys, that’s gross. What the hell?” he burst out. Sam and Dean exchanged nervous looks. “Why would you kiss your own brother? I need to burn that from my eyes. Eww!” Sam squinted at Jimmy noticing a smile pulling at the corner of his mouth. “Blasphemous, dirty bad wrong and just-” he laughed aloud now. “So damn hot too! Can I watch? Because seriously. I’m gonna need a- ow!”

Dean walked up and punched him on the shoulder. “I thought you were serious!”

Jimmy rubbed his shoulder. “Come on. You guys were all straight faced with that. I had to break the tension some how. So brothers, really? We should have all come clean from the start. Would have made all our lives easier. Cas and I have had to be so quiet!”

Sam opened his mouth to reply but there was a knock on the door.

Dean opened it.

“Hello Dean,” Cas said, stepping inside. “Hi Sam!” She smiled brightly and waved.

“We were just talking about you,” Jimmy said wrapping his arms around her waist.

She melted against him, tucking her head under his chin. “Good things, I should hope.” She peaked at Sam and he smiled. There was a twinge of jealousy. For both of them, he realized but with Dean right beside him it was easy to let go.

“Uh huh. Can I tell her?” Jimmy asked over her head.

Sam nodded.

“Tell me what?” Cas asked pulling away enough to look from Jimmy to Sam.

“Well, I was just saying how it’s nice we don’t have to hide anymore. At least in front of Sam and Dean. They were surprisingly understanding. Wanna know why?”

Cas frowned. “It’s not just because they are understanding people?”

“They are.” Jimmy paused for dramatic effect and Sam wanted to rolled his eyes. “But it helps that they are brothers.”

Cas looked from Sam to Dean and back. “Really?”

Sam nodded. “Yeah, sorry for not telling you.”

“Oh, it’s okay.” She considered them for a moment longer, tilting her head to one side. “I guess I can see the resemblance.” She shrugged. “Anyway, I wanted to ask you something Sam. Would you like to accompany me to dinner?”

Sam smiled at the abrupt change in topic but remembered, even if Dean was alright with it, they hadn’t talked to Jimmy about this yet. Sam looked at her brother.

“Oh you too lovebirds have go fun. I’m sure Dean and I can entertain ourselves here. Just some pizza and movies for us, right Dean?”

Dean grinned in a way Sam was all too familiar with. “Yeah, sounds good.”

“Sam?” Cas asked, getting his attention.

“Yeah Cas, sounds great. There’s an Italian place I’ve been wanting to go to.” He held out his hand for hers.

She took it smiling. She looked so happy she was practically glowing. Sam couldn’t help smile back. How had he ever thought he wasn’t head of heels for her?

The both kissed their brothers good bye, not letting go of each other's hands, making their arms stretch between them.

“Jimmy thinks Dean is hot,” Cas said as the door closed behind them. “And he’s curious if Dean is good in bed.”

Sam laughed. “Yes,” he answered. They started down the hall.

“Better than Jimmy?” she asked after a pause.

Sam shrugged. “I wouldn’t know.”

“We should find out sometime.” She said it so sweetly and matter of factly Sam wanted to kiss her.

Sam also noted that she said ‘we.’

“We should. After dinner though.”

Looked at him like he was being ridiculous. “Of course.”

**Author's Note:**

> Come say hi on Tumbler! I go by [Jerksarehot](https://jerksarehot.tumblr.com/) over there.


End file.
